In casting, impurities such as slag contaminated in a molten metal, if it remains through a final product, can cause cast defects in the product. There are various causes of contamination of impurities such as oxidation of a material to be molten and a molten metal, a fallen part of a mold and contamination of a mold material. It is in fact almost impossible to avoid the contamination. In practical operation, efforts for reducing the contamination as small as possible and for avoiding the contamination in a product by devising a process of casting are generally made. One of such efforts is a method of arranging a filter made of a fire-resistant material such as ceramics in a runner system including a sprue, a runner and a gate to remove impurities from a molten metal. This method is often employed, because it has high reliability.
This method, however, cannot use a filter having so small mesh due to limitation of running resistance, and thus is effective for removing relatively large impurities such as slug, but not for small impurities such as mold sand. In the case of producing a cast article particularly disliking a defect due to contamination of impurities, employed is a method of using a runner tube made of a fire-resistant material in a runner system to avoid contamination of sand from a mold and removing slug and the like from a molten metal with a filter. However, it is difficult to set a runner tube and a filter on uncured mold sand to form a mold in forming a mold, because the uncured sand is unstable and not suitable for positioning. This method may further cause other defects such as break of the filter and contamination of sand in the runner tube.
To solve these problems, those are proposed, including a molded molten metal passage having a filter set part integrally formed (JP-Y2-30117), a mold having a sprue and a filter which are integrated with a fire-resistant sleeve (JP-A1-224139) and a sintered fire-resistant filter holder for molten metal having a construction of connecting a runner and holding a filter (JP-U5-9736). JP-A-2004-181472 discloses a mold or a structure for producing a cast article, containing organic and inorganic fibers and a thermosetting resin. In this patent, there is no description about a filter holder nor an object of this invention.